Optical fiber devices and methods for reducing stimulated Raman scattering (SRS) light emissions from a resonant cavity

Abstract

Fiber laser devices, systems, and methods for reducing Raman spectrum in emissions from a resonant cavity. A fiber laser oscillator that is to generate an optical beam may include a Raman reflecting output coupler that strongly reflects a Raman component pumped within the resonant cavity, and partially reflects a signal component to sustain the oscillator and emit a signal that has a reduced Raman component. A Raman filtering output coupler may comprise a superstructure fiber grating, and such a grating may be chirped or otherwise designed to have a desired bandwidth.

What is claimed is: 
     
       1. A fiber laser oscillator to generate a light beam, the fiber laser oscillator comprising:
 a length of optical fiber comprising a core and one or more cladding layers, wherein at least a portion of the length of the optical fiber is doped with a gain medium operable to excite at least a signal component of the light beam, the signal component associated with a first peak wavelength; 
 a first reflector proximal to a first end of the length of optical fiber; and 
 an output coupler separated from the first reflector by at least the length of optical fiber, wherein the output coupler has a lower reflectivity at the first peak wavelength than at a second peak wavelength associated with a Raman component of the light beam, 
 wherein:
 the first reflector has a higher reflectivity at the first peak wavelength than at the second peak wavelength; 
 the Raman component is to propagate in a fundamental core mode of the length of optical fiber; and 
 the output coupler comprises a superstructure fiber grating (SS-FG) that is to reflect the Raman component into a counter-propagating core mode of the length of optical fiber. 
 
 
     
     
       2. The fiber laser oscillator of  claim 1 , wherein the first reflector has a narrower bandwidth than the output coupler. 
     
     
       3. The laser oscillator of  claim 1 , wherein the SS-FG comprises one or more chirped refractive index modulations having a periodicity, or magnitude of refractive index modulation, that varies over a length of the SS-FG. 
     
     
       4. The fiber laser oscillator of  claim 3 , wherein the chirped refractive index modulations have a first end with a shortest grating period proximal to the length of optical fiber, and a second end with a longest period proximal to a second length of optical fiber that is separated from the first length of optical fiber by the SS-FG. 
     
     
       5. The fiber laser oscillator of  claim 1 , wherein the SS-FG has a single reflectivity peak within a Raman band. 
     
     
       6. The fiber laser oscillator of  claim 1 , wherein the SS-FG has a plurality of reflectivity peaks within a Raman band. 
     
     
       7. The fiber laser oscillator of  claim 1 , wherein:
 at least one of the first or second peak wavelengths is to be between 1000 nm and 1200 nm; 
 the second peak wavelength is longer than the first peak wavelength by at least 40 nm; and 
 the one or more cladding layers of the length of optical fiber further comprise an inner cladding layer, and an outer cladding layer in contact with the inner cladding layer. 
 
     
     
       8. The fiber laser oscillator of  claim 7 , wherein the length of optical fiber supports only one core propagation mode and the SS-FG comprises refractive index modulations within the core. 
     
     
       9. A fiber laser system, comprising:
 the fiber laser oscillator of  claim 1 ; and 
 a fiber power amplifier coupled to receive a portion of the signal component through the output coupler. 
 
     
     
       10. The system of  claim 9 , further comprising
 a delivery fiber coupled to the fiber power amplifier; and 
 a process head coupled to the delivery fiber to launch an optical beam into free space. 
 
     
     
       11. A fiber laser system, comprising:
 a fiber laser oscillator to generate a light beam, wherein the fiber laser oscillator comprises:
 a length of optical fiber comprising a core and one or more cladding layers, wherein at least a portion of the length of optical fiber is doped with a gain medium operable to excite at least a signal component of the light beam, the signal component associated with a first peak wavelength; 
 a first reflector proximal to a first end of the length of optical fiber, wherein the first reflector has a higher reflectivity at the first peak wavelength than at the second peak wavelength; and 
 an output coupler separated from the first reflector by at least the length of optical fiber, wherein the output coupler has a lower reflectivity at the first peak wavelength than at a second peak wavelength associated with a Raman component of the light beam, wherein: 
 the first reflector has a higher reflectivity at the first peak wavelength than at the second peak wavelength; 
 the Raman component is to propagate in a fundamental core mode of the length of optical fiber; and 
 the output coupler comprises a superstructure fiber grating (SS-FG) that is to reflect the Raman component into a counter-propagating core mode of the length of optical fiber; and 
 a fiber power amplifier coupled to receive a portion of the signal spectrum through the output coupler. 
 
 
     
     
       12. A method of generating a light beam, the method comprising:
 energizing a fundamental propagation mode of light within a first length of optical fiber, wherein the first length of optical fiber comprises a core doped with an optical gain media and at least one cladding layer, and wherein the light comprises a signal component associated with a first peak wavelength, and a Raman component associated with a second peak wavelength; 
 reflecting the light into the fundamental mode with a first reflector at a first end of the first length of optical fiber; 
 reflecting the light into the fundamental mode with a second reflector at a second end of the first length of optical fiber, wherein the second reflector has a higher reflectivity at the second peak wavelength than the first peak wavelength, the second reflector comprises a superstructure fiber grating (SS-FG), and the first reflector has a higher reflectivity at the first peak wavelength than at the second peak wavelength; 
 propagating in a second length of optical fiber a fraction of energy associated with the signal component that is transmitted through the second reflector; and 
 dumping energy associated with the Raman component that is transmitted through the first reflector. 
 
     
     
       13. The method of  claim 12 , wherein the first reflector has a narrower bandwidth than the second reflector. 
     
     
       14. The method of  claim 12 , wherein the SS-FG comprises one or more chirped refractive index modulations having a periodicity that varies, with a shortest grating period proximal to the first length of optical fiber, and longest period proximal to the second length of optical fiber. 
     
     
       15. The method of  claim 12 , wherein the SS-FG has a plurality of reflectivity peaks within a Raman band. 
     
     
       16. The method of  claim 12 , wherein:
 at least one of the first or second peak wavelengths is between 1000 nm and 1200 nm; 
 the second peak wavelength is longer than the first peak wavelength by at least 40 nm; 
 the at least one cladding layer comprises an inner cladding layer, and an outer cladding layer in contact with the inner cladding layer; 
 the signal component is to have an optical power that is at least 1 kW; and 
 the first length of optical fiber supports only one propagation mode.